Oxy-calcination process

ABSTRACT

Method and installation for calcining cement raw meal in a calciner whereby fuel and a calciner oxidant having an oxygen content of at least 30% vol are introduced into the calciner so as to generate either an oxidant-lean zone or a fuel-lean zone in the calciner located between the lowermost fuel inlet level and the lowermost oxidant inlet level of the calciner, between 50% and 100% by weight of the raw meal being supplied to the calciner upstream of and/or within the oxidant-lean, respectively the fuel-lean zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/532,325, filed Jun. 1, 2027, which is a § 371 of International PCTApplication PCT/EP2015/078100, filed Nov. 30, 2015, which claims §119(a) foreign priority to EP patent application EP14306933.4, filedDec. 1, 2014, all of which are incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to the calcination of raw cement mealusing oxy-combustion, the calcination of raw meal being an essentialstep in the production of cement clinker.

Related Art

The cement industry is an important emitter of the greenhouse gas CO₂.

Within the cement production process, significant amounts of CO₂ aremore particularly generated during the decarbonation of raw meal (CaCO₃)to lime (CaO) via the following reversible equilibrium reaction:

${{CaCO}_{3}\mspace{14mu} \overset{K}{\;}\mspace{11mu} \text{CaO}} + {CO}_{2}$Δ H ≈ 1800  kJ/kg${K = {A\; {\exp \left( \frac{- E_{a}}{RT} \right)}}},$

so that about 80% of the CO₂ generated by a cement plant is produced atcalciner level.

As explained in the article “The oxycombustion option” published in theMay 2014 issue of the INTERNATIONAL CEMENT REVIEW 37, the cementindustry has made considerable efforts to lower its CO₂ emissionsthrough the use of alternative fuels, lower specific heat consumption inkiln systems and a decrease of the clinker factor with the addition ofsupplementary cementitious materials leading to CO₂ reduction by 20-30%.

A possible route for further CO₂ mitigation lies in the application ofcarbon capture and storage technology (CCS) or carbon capture, storageand utilization technology (CCSU). This entails capturing CO₂ from thecement plant's flue gases for storage or for use in other industrialapplications.

The air used in conventional combustion processes consists mainly ofnitrogen (about 78% vol), said nitrogen also forming the mainconstituent of the flue gas generated by air-combustion.

Several technologies have been developed to extract and capture CO₂ fromsuch flue gases, in particular for the power industry.

The current reference technology for capturing CO₂ present in flue gasesis amine scrubbing.

This process consists of extracting the CO₂ fraction from apost-combustion flue gas by flushing the gases with an amine sorbent,regenerating the solvent by steam stripping, thus releasing nearly pureCO₂, and recycling the stripped solvent to the absorber. Although thistechnology is very efficient, it is also quite expensive.

An alternative to post-combustion amine scrubbing is the use ofoxycombustion.

In the oxycombustion process oxygen and recycled flue gas replace theconventional combustion air, so as to directly generate a CO₂-rich fluegas during combustion and thereby to reduce downstream CO₂ purificationcosts.

In a cement plant, oxycombustion can be applied either to the fullproduction line (i.e. in both the calciner and rotary kiln section),such a process being referred to as “full oxy-firing”, or solely at thecalciner stage, referred to as “partial oxy-firing”. In the comparativestudy “CO₂ Capture in the cement Industry”, Report no 2008/3, publishedby the International Energy Agency (IEA), it was concluded that partialoxy-firing is the most cost-effective and lowest-risk configuration forretrofitting an existing cement plant. The IEA report also concludedthat partial oxy-firing was cheaper than post-combustion amine scrubbingtechnology.

However, operating a calciner in oxycombustion mode has a major impacton the abovementioned decarbonation reaction because of the increase inCO₂ partial pressure. Indeed, the desired decarbonation reaction onlyoccurs if the equilibrium pressure—which strongly depends on thetemperature—exceeds the surrounding CO₂ partial pressure.

In order to counteract the high CO₂ content linked to oxycombustion, itwould therefore be necessary to operate the calciner at higher averagetemperature.

In calciners operating with air-combustion, the atmosphere containstypically from 25% vol to a maximum of 35% vol CO₂. The correspondingequilibrium temperature of the decarbonation reaction is in the range of800° C. to 850° C. According to the abovementioned study “CO₂ Capture inthe cement Industry”, the switch from air-combustion to oxy-combustionin a calciner would require a calciner temperature increase of around80° C. to compensate for the increase in CO₂ partial pressure.

As recognized in the article “The oxycombustion option”, operating thecalciner at higher average temperatures entails an increased risk ofhotspots within the calciner, even more so as burning fuel with oxygenis known to generate high-temperature product gases.

Such hotspots are responsible for disruptive material build-ups withinthe calciner leading to costly calciner shutdowns, the alternative beingto operate the calciner at lower temperature, which would result in asignificant deterioration of the process efficiency (lower calcinationdegree). This problem is even more manifest when fuels, such as petcoke,are used which require high temperatures and high residence times inorder to achieve substantially complete combustion. Such fuels arefrequently used in the cement industry in order to lower productioncosts.

SUMMARY OF THE INVENTION

It is an aim of the present invention to at least partially overcome theabovementioned problems.

The invention aims in particular to provide a method of calcining cementraw meal in a calciner using oxy-combustion and which permitssimultaneously to achieve a sufficient level of calcination and toreduce or even to avoid the occurrence of detrimental build-ups in thecalciner.

The calciner extends between a bottom end and a top end in alongitudinal direction, said longitudinal direction being typicallyvertical or substantially vertical.

Fuel and calciner oxidant are introduced into the calciner. The fuel isburnt with the calciner oxidant to generate heat inside the calciner.The calciner oxidant has an oxygen content of between 30 and 100% vol.

The calciner oxidant advantageously has an oxygen content of at least50% vol, preferably of at least 88% vol.

The fuel and calciner oxidant are introduced into the calciner so as toensure substantially complete and preferably complete combustion of saidfuel and, preferably, so as to minimize excess oxygen in the flue gas atthe calciner outlet, taking into account any air ingress into thecalciner. The amount of oxygen, referred to as excess oxygen, present inthe calciner flue gas is typically maintained below 7% vol, preferablybelow 5% vol.

Fuel combustion is said to be substantially complete when the organiccarbon content of the calcined meal is less than 0.5% by weight.

Raw meal is likewise supplied to the calciner. Within the calciner, theraw meal is entrained towards the top end by an upward gas flow.

The raw meal is calcined in the calciner and the calcined meal thusobtained is evacuated from the calciner at the top end together with thecalciner flue gas.

-   -   The upward gas flow which entrains the raw meal typically        comprises: the flue gases generated by the burning of the fuel        with the calciner oxidant    -   the decarbonation gas generated by the calcination of the raw        meal and which consists essentially of CO₂ and    -   part of the calciner flue gas which is introduced into the        calciner via the bottom end as recycle flue gas.

Once evacuated from the calciner, the calcined meal is separated fromthe calciner flue gas. Part of the separated calciner flue gas is, asdescribed above, introduced into the calciner as the recycle flue gas.

The calciner oxidant is introduced into the calciner at at least oneoxidant inlet level. The fuel is introduced into the calciner at atleast one fuel inlet level.

The at least one oxidant inlet level may consist of a single oxidantinlet level which is then referred to as “the lowermost oxidant inletlevel”.

Alternatively, the at least one oxidant inlet level may consist ofmultiple oxidant inlet levels along the longitudinal direction of thecalciner, the oxidant inlet level nearest to the bottom end of thecalciner being then referred to as “the lowermost oxidant inlet level”.

Likewise, the at least one fuel inlet level may consist of a single fuelinlet level which is then referred to as “the lowermost fuel inletlevel”.

Alternatively, the at least one fuel inlet level may consist of multiplefuel inlet levels along the longitudinal direction of the calciner, thefuel inlet level nearest to the bottom end of the calciner being thenreferred to as “the lowermost fuel inlet level”.

In accordance with the present invention, between 50% and 100% by weightof the raw meal is supplied to the calciner in a zone of limited or evenno fuel combustion.

This is achieved according to one of the following options:

option 1: the lowermost oxidant inlet level is located downstream (interms of the upward gas flow) of the lowermost fuel inlet level and thisat an oxygen-lean zone distance Do(>0) from the lowermost fuel inletlevel thereby creating an oxygen-lean zone in the calciner, at least 50%by weight of the raw meal being supplied to the calciner upstream ofand/or at the lowermost oxidant inlet level, preferably at least 85% byweight of the raw meal;

or

option 2: the lowermost fuel inlet level is located downstream (in termsof the upward gas flow) of the lowermost oxidant inlet level and this ata fuel-lean zone distance Df(>0) from the lowermost oxidant inlet levelthereby creating a fuel-lean zone in the calciner, at least 50% byweight of the raw meal being supplied to the calciner upstream of and/orat the lowermost fuel inlet level, preferably at least 85% by weight ofthe raw meal.

Option 1 is generally preferred.

When fuel or calciner oxidant is mixed with the recycle flue gas beforethe latter is introduced into the calciner, for example inside the riserduct, said fuel, respectively calciner oxidant, is entrained by therecycle flue gas and enters the calciner via the bottom end togetherwith the recycle flue gas. In that case, the level of the calcinerbottom end corresponds to the lowermost fuel inlet level, respectivelythe lowermost oxidant inlet level.

In the context of the description of the present invention, the terms“downstream” and “upstream” are to be interpreted with respect to theupward gas flow in the calciner. “Downstream” thus refers to a higherlevel in the calciner (as seen in the longitudinal direction) and“upstream” to a lower level in the calciner.

In the method according to the first option, at least 50% by weight ofthe raw meal is advantageously supplied to the calciner upstream of thelowermost oxidant inlet level, preferably at least 75% by weight or evenat least 85% by weight.

Likewise, in the method according to the second option at least 50% byweight of the raw meal is advantageously supplied to the calcinerupstream of the lowermost fuel inlet level, preferably at least 75% byweight or even at least 85% by weight.

It was surprisingly found that, whereas detrimental build-ups ofmaterial were rapidly observed within the calciner for otherconfigurations of meal, fuel and calciner oxidant introduction into thecalciner, this was not the case for the configurations of meal, fuel andcalciner oxidant introduction in accordance with the invention.

It was more specifically found that, with the configurations of meal,fuel and calciner oxidant introduction of the invention, the higher theportion of raw meal injected at and/or (preferably) upstream of thelowermost oxidant inlet level (option 1), respectively fuel inlet level(option 2), the more the formation of build-ups in the calciner wasinhibited. Typically, the raw meal introduced into the calciner consistsat least in part of raw meal preheated in a preheater. Preferably all ofthe raw meal introduced into the calciner has been preheated in apreheater. Further details regarding how raw meal may be preheated areprovided below.

The oxygen-lean zone distance Do, respectively the fuel-lean zonedistance Df is advantageously between 1/10 and 4/10 of the totalcalciner height, said distance being more preferably between 2/10 and3/10 of the total calciner height.

The oxygen-lean zone, respectively the fuel-lean zone is advantageouslylocated in the lowermost half, preferably in the lowermost third of thecalciner, i.e. in the half or third of the calciner including the bottomend.

One or more than one oxidant inlets maybe located at the lowermostoxidant inlet level.

When at the lowermost oxidant inlet level, at least part of the calcineroxidant is introduced through a plurality of oxidant inlets, saidoxidant inlets being hereafter referred to as “first oxidant inlets”,then said first oxidant inlets are advantageously radially spaced apartfrom one another around the longitudinal direction of the calciner andare preferably evenly distributed around said longitudinal direction.

When at the lowermost fuel inlet level, fuel is introduced into thecalciner via a multitude of fuel inlets, then said “first fuel inlets”are likewise advantageously radially spaced apart from one anotheraround the longitudinal direction, preferably evenly distributed aroundsaid longitudinal direction.

In many instances, the calciner operation can be improved and the riskof detrimental material build-ups can be further reduced by introducingcalciner oxidant at multiple oxidant inlet levels, in particular inconnection with option 1 of the present invention. In this manner, heatgeneration by combustion in the calciner is staged.

In that case, the calciner oxidant is divided in a first portion and asecond portion of calciner oxidant. The first portion of calcineroxidant is introduced into the calciner at the lowermost oxidant inletlevel through one or more first oxidant inlets. The second portion ofthe calciner oxidant is introduced into the calciner at one or moresecond oxidant inlet levels above the lowermost oxidant inlet levelthrough one of more “second oxidant inlets” at each second oxidant inletlevel.

When the second portion of calciner oxidant is introduced into thecalciner through multiple second oxidant inlets:

-   -   at least some of said second oxidant inlets may be spaced apart        from one another in the longitudinal direction of the calciner,        i.e. may be located at different second oxidant inlet levels;        and/or    -   at least some of said second oxidant inlets may be spaced apart        from one another radially, i.e. with multiple second oxidant        inlets at a given second oxidant inlet level being radially        spaced apart around said longitudinal direction, preferably        evenly so.

The considerations presented above with respect to the calciner oxidantalso apply to the fuel that is supplied to the calciner when said fuelintroduced into the calciner at multiple fuel inlet levels, inparticular in the case of the embodiment according to option 2.

It will be appreciated that a cement production installation may alsocomprise additional equipment such as a second preheater. For example,in a second raw meal preheater, the raw meal can be preheated by meansof flue gas coming from the rotary kiln. The process according to thepresent invention requires that at least one, but not necessary all,calciners is operated as described above.

The fuel introduced into the calciner may include a combination ofdifferent types or qualities of fuel.

Compared to calciners operated with air as the calciner oxidant, themethod according to the present invention directly generates a CO₂-richflue gas which leaves the calciner at its top end. As a result, therecycle flue gas contains at least 40% by dry volume of CO₂, preferablyat least 60% and more preferably at least 75% by dry volume.

The portion of the calciner flue gas which is introduced into thecalciner as recycle flue gas depends inter alia on the amount of fluegas which is required to generate an upward gas flow in the calcinerwhich is sufficient for entraining the raw meal to the top end of thecalciner while also ensuring a residence time of the fuel sufficient toachieve complete or substantially complete fuel combustion. In practice,achieving proper burn-out of the fuel within the calciner is often themost critical factor and the amount of recycle flue gas isadvantageously at or near the minimum level of recycle flue gasnecessary for entraining the raw meal to the top end of the calciner.

In doing so, the residence time of the raw meal in the calciner isgenerally such that the required level of decarbonation is reached. Therecycle flue gas typically corresponds to between 10% vol and 80% vol,preferably between 30% vol and 50% vol of the total calciner flue gas.

The calciner flue gas evacuated from the top end of the calciner isadvantageously introduced into a raw meal preheater, such as, forexample, a single or multistage cyclone preheater, before part of it isrecycled to the calciner as recycle flue gas.

It will be appreciated that the cement production installation maycomprise additional equipment such as a second preheater.

The recycle flue gas is usefully introduced into the calciner at atemperature of at least 400° C., preferably of at least 700° C. and morepreferably of at least 900° C. As a consequence, in particular when thecalciner flue gas has gone through a raw meal preheater before part ofit is recycled, resulting in a lowering of the flue gas temperature, therecycle flue gas may itself be (pre)heated before being introduced intothe calciner.

The portion of the calciner flue gas which is not recycled to thecalciner is typically subjected to a purification process to extractconstituents other than CO₂ therefrom so as to enable the valorizationor sequestration/storage of the CO₂ present within said non-recycledportion of the calciner flue gas, thereby reducing the CO₂ emissions ofthe cement production process.

It is an advantage of the present invention that largely pure CO₂ can beobtained from the non-recycled portion of the calciner flue gas usingpurification methods other than the relatively expensive amine scrubbingtechnology. As indicated above, following the purification process, thepurified calciner flue gas may be stored and/or used as CO₂ in anindustrial process.

The present invention also relates to the use of the present method ofcalcining cement raw meal in the production of cement clinker.

The present invention thus also covers a method of producing cementclinker whereby raw meal is calcined by the method according to theinvention and whereby the calcined meal is introduced into a kiln andsubjected to clinkerisation within the kiln, the kiln being typically arotary kiln.

According to one embodiment of the cement production method according tothe invention (a) the kiln gas outlet is not connected to the bottom endof the calciner but for example to another preheater tower and (b)combustion takes place in the kiln with a kiln oxidant having an oxygencontent of less than 30% vol, such as air and in particular air comingfrom a clinker cooler.

According to an alternative embodiment combustion takes place in thekiln with a kiln oxidant having an oxygen content of between 30% vol and100% vol, preferably of at least 50% vol and more preferably of at least88% vol. In this case the kiln gas outlet may be connected to the bottomend of the calciner. In particular, all or part of the calciner flue gaswhich is introduced into the calciner as recycle flue gas may then firstbe injected into the kiln, for example via the clinker cooler, andthereafter introduced into the calciner as part of the kiln flue gas. Inthis manner, the recycle flue gas is also preheated during its passagethrough the clinker cooler.

The present invention also relates to a calcination installation for usein the calcination method according to the invention. Such a calcinationinstallation for calcining cement raw meal comprises a calciner whichhas a total calciner height and which extends, in a longitudinaldirection, between a bottom end and a top end.

The calciner of the calcination installation of the invention presents alowermost oxidant inlet level at which one or more first oxidant inletsare located, and optionally one or more second oxidant inlet levelslocated above the lowermost oxidant inlet level in the longitudinaldirection, one or more second oxidant inlets being located at eachsecond oxidant inlet level present. The first oxidant inlets and, ifpresent, the second oxidant inlets of the calciner are connected to asource of calciner oxidant having an oxygen content of at least 30% vol,preferably at least 50% vol and more preferably at least 88% vol, sothat calciner oxidant can be supplied to said oxidant inlets and beinjected into the calciner via said oxidant inlets.

The calciner also presents a lowermost fuel inlet level at which one ormore fuel inlets are located, and optionally one or more second fuelinlet levels located above the lowermost fuel inlet level in thelongitudinal direction, whereby one or more further fuel inlets,referred to as “second fuel inlets” are located at each second fuelinlet level present.

In addition, the calciner presents a flue gas outlet located at the topend of the calciner, a flue-gas recycle inlet located at the bottom endof the calciner and one or more raw meal inlets.

According to the present invention, the configuration of the meal, fueland calciner oxidant inlets of the calciner is such that, in operation,a low-combustion zone is created within the calciner and so that atleast part of the meal is introduced into the calciner at saidlow-combustion zone.

According to a first option, this is achieved in that the lowermostoxidant inlet level is located above the lowermost fuel inlet level atan oxygen-lean zone distance Do>0 (in the longitudinal direction) fromsaid lowermost fuel inlet, at least one raw meal inlet being locatedbelow or at the lowermost oxidant inlet level, preferably below.

According to a second option, this is achieved in that the lowermostfuel inlet level is located above the lowermost oxidant inlet level at afuel-lean zone distance Df>0 (in the longitudinal direction) from thelowermost oxidant inlet level, at least one raw meal inlet being locatedbelow or at the lowermost fuel inlet level.

The oxygen-lean zone distance Do, respectively the fuel-lean zonedistance Df, is advantageously between 1/10 and 4/10 of the totalcalciner height or even more preferably between 2/10 and 3/10 of thetotal calciner height.

In operation, the flue gas outlet of the calciner is connected to theflue-gas recycle inlet of the calciner via a recycle circuit so as toenable part of the calciner flue gas to be introduced into the calcinervia the flue-gas recycle inlet as recycle flue gas. As described abovewith respect to the calcination method, the recycle circuit mayincorporate devices such a raw meal preheater and may as describedabove, even include a kiln when said kiln is operated with a kilnoxidant containing at least 30% vol of oxygen.

The calcination installation of the invention may thus comprise a rawmeal preheater connected to the flue gas outlet, so as to receivecalciner flue gas therefrom, and to the one or more raw meal inlets ofthe calciner, so as to provide preheated raw meal thereto.

In order to enable CO₂ valorisation or storage/sequestration, thecalcination installation preferably comprises a flue gas purificationinstallation connected to the calciner flue gas outlet, said flue gaspurification installation being adapted for removing components otherthan CO₂ from flue gas evacuated from the calciner via the flue gas.When a raw meal preheater is connected to the flue gas outlet so as toreceive calciner flue gas therefrom, the flue gas purificationinstallation is preferably connected to the raw meal preheater so as toreceive calciner flue gas from the raw meal preheater, the flue gaspurification installation being thus indirectly connected to thecalciner flue gas outlet.

The present invention also relates to a cement clinker production unitcomprising a calcination installation as described above and aclinkerisation kiln, typically a rotary clinkerisation kiln. Theclinkerisation kiln is connected to the calcination installation so thatmeal calcined in the calciner is transferred to the clinkerisation kilnso as to be clinkerized therein.

BRIEF DESCRIPTION OF THE FIGURES

The present invention and its advantages will be better understood inthe light of the examples below, reference being made to FIGS. 1 and 2,whereby:

FIG. 1 is a partial schematic representation of an installation suitablefor use in the calcination method according to option 1 of the presentinvention, and

FIG. 2 is a partial schematic representation of an installation suitablefor use in the calcination method according to option 2 of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a calciner 10 extending, in its vertical longitudinaldirection, between a bottom end 11 and a top end 12, the calciner havinga total height H.

In the illustrated example, all of the fuel 13 is introduced into ariser duct 19 upstream of bottom end 11. In the present case, the solidfuel 13 is petcoke, but a combination of different fuels, includingwaste fuels and/or fossil fuels, may be introduced, for example atdifferent locations of the calciner 10.

The illustrated calciner 10 thus has only a single fuel inlet level,i.e. the lowermost inlet level L13, which coincides with the (level ofthe) bottom end 11 of the calciner, the riser duct connection to thebottom end 11 of the calciner acting as the single fuel inlet L13 intothe calciner 10.

All of the calciner oxidant 16 is introduced into the calciner 10 atlevel L16 via multiple oxidant inlets evenly distributed around thecircumference of the calciner 10.

The calciner oxidant 16 has an oxygen content of 99% vol. All of thecalciner oxidant 16 is injected into the calciner 10 at level L16, levelL16 thus being the sole and lowermost oxidant inlet level of thecalciner, the oxidant inlets at level L16 being first oxidant inlets asdefined above. Via said first oxidant inlets, the calciner oxidant isintroduced into the calciner 10 in an amount sufficient to ensurecomplete combustion of the overall amount of fuel, while minimizing anyexcess oxygen in the calciner flue gas.

Do is the oxygen-lean zone distance between the upstream lowermost fuelinlet level L13 and the downstream lowermost oxidant inlet level L16.The zone in the calciner 10 between the lowermost fuel inlet level L13and lowermost oxidant inlet level L16, i.e. between bottom end 11 andlevel L16, is an oxygen-lean zone in which little or no fuel combustiontakes place.

A first portion 14 of (preheated) raw meal is injected into riser duct19 upstream of calciner 10. This first portion 14 of the raw meal isthus introduced into the calciner 10 via its bottom end 11 where itenters said oxygen-lean zone. The remainder 15 of the (preheated) rawmeal is injected into the oxygen-lean zone of calciner 10 via a raw mealinlet positioned at level L15 downstream of the lowermost fuel inletlevel L13 and upstream of the lowermost oxygen inlet level L16.

The raw meal 14, 15 introduced into the calciner 10 is entrained towardsthe top end 12 of the calciner 10 by an upward gas flow. During itsupward passage through the calciner 10, the raw meal is at leastpartially, and in fact for at least 92%, calcined under the influence ofheat generated by the burning of the fuel 13 with the calciner oxidant16 and by any heat introduced into the calciner 10 by means of recycleflue gas. In the present context partially and totally calcined cementmeal leaving the calciner are indiscriminately referred to as “calcinedmeal”. The calcined meal is evacuated from the calciner 10 via its topend 12 together with the calciner flue gas.

The upstream gas stream which entrains the raw meal comprises fumesgenerated by the burning of the fuel 13 and decarbonation gas (CO₂)generated by the decarbonation of the raw meal 14, 15. As will beexplained hereinbelow, the upstream gas stream further comprises recycleflue gas.

From the top end 12 of the calciner 10, the calciner flue gas and thecalcined meal are transported to a first cyclone, separation cyclone 20,in which the calcined meal 21 is separated from the calciner flue gas.

From separation cyclone 20, the separated calcined meal 21 is typicallytransported to a rotary clinkerization kiln (not represented) for theproduction of clinker. In the case of partial oxy-firing, combustiontakes place in the rotary kiln with an oxidant having an oxygen contentof less than 30% vol. From separation cyclone 20, the separated calcinerflue gas is introduced into raw meal preheater tower 30 which comprisesthree further cyclones 31, 32, 33 through which the separated calcinerflue gas flows in succession. Raw meal 40 to be preheated is introducedin the gas outlet of middle cyclone 32 via inlet 34 from where it isentrained by the gas flow into top cyclone 33. From top cyclone 33, thepartially preheated raw meal is introduced into the gas outlet of bottomcyclone 31 (of the preheater tower 30) via inlet 35 from where it isentrained to middle cyclone 32. From middle cyclone 32, the raw meal isintroduced into the gas outlet of separation cyclone 20 via inlet 36from where it is entrained to bottom cyclone 31 of tower 30, whereafterthe preheated raw meal is sent to meal splitter 50 before beingintroduced into calciner 10 as described above, i.e. indirectly viariser duct 19 and directly into calciner 10 at level L15.

Extractor fan 60 extracts the calciner flue gas from top cyclone 33.Downstream of extractor fan 60 the calciner flue gas is split into twostreams, a first stream 70, which is removed from the system and sentfor downstream flue gas processing, and a second stream 80 of recycleflue gas which is recycled and introduced into calciner 10 via riserduct 19 and bottom end 11, as part of the upward gas flow in thecalciner. If appropriate, the recycle flue gas 80 can be preheatedbefore being reintroduced into calciner 10 (not illustrated). In thegiven example, the recycle flue gas 80 was introduced into the calcinerat a temperature of 800 to 900° C.

As mentioned above, the cement production installation may comprise asecond raw meal preheater (not shown), which is, for example, fed withthe flue gas of the rotary kiln.

FIG. 2 shows an installation similar to the one shown in FIG. 1, but inwhich cement raw meal is calcined using the method of calcining cementraw meal according to the second option of the invention.

In the illustrated example, all of the calciner oxidant 16′ isintroduced into the riser duct 19 upstream of bottom end 11. Calciner 10thus has only a single oxidant inlet level, i.e. the lowermost inletoxidant level L16′, which coincides with the bottom end 11 of thecalciner 10, the riser duct connection to the bottom end 11 of thecalciner acting as the single oxidant inlet into the calciner 10.

All of the fuel 13′ is introduced into the calciner 10 at level L13′ viaa single fuel inlet or via multiple fuel inlets evenly distributedaround the circumference of the calciner 10. Level L13′ is thus the soleand lowermost fuel inlet level of the calciner.

Df is the fuel-lean zone distance between the upstream lowermost oxidantinlet level L16′ and the downstream lowermost fuel inlet level L13′,i.e. between the bottom end 11 and the lowermost fuel inlet level L13′.The zone in the calciner 10 between the bottom end 11 of the calciner 10and level L13′ is a fuel-lean zone in which no fuel combustion takesplace.

Again, a first portion 14 of (preheated) raw meal is injected into riserduct 19 upstream of calciner 10. This first portion 14 of the raw mealis thus introduced into the calciner 10 via its bottom end 11 where itenters said fuel-lean zone. The remainder 15 of the (preheated) raw mealis injected into the oxygen-lean zone of calciner 10 via a raw mealinlet positioned at level L15 downstream of the lowermost oxidant inletlevel L16′ and upstream of the lowermost fuel inlet level L13′.

Apart from the above, the process and installation features areanalogous to those of the previous example.Both options of the methodaccording to the invention enable operation of the calciner 10 withoutany deterioration of the process due to material build-ups inside saidcalciner 10 while maintaining a high level of calcination. When,however, under otherwise similar process conditions, fuel, calcineroxidant and raw meal were introduced into calciner 10 with aconfiguration of the meal, fuel and calciner oxidant inlets andinjection ratios known from the state of the art and not correspondingto a configuration according to the present invention, the processefficiency started to deteriorate within a few hours of operation due toincreasing levels of material build-up in the calciner 10.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing i.e.anything else may be additionally included and remain within the scopeof “comprising.” “Comprising” is defined herein as necessarilyencompassing the more limited transitional terms “consisting essentiallyof” and “consisting of”; “comprising” may therefore be replaced by“consisting essentially of” or “consisting of” and remain within theexpressly defined scope of “comprising”.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

What is claimed is:
 1. A calcination installation for calcining cementraw meal, the installation comprising a calciner having a total calcinerheight and extending between a bottom end and a top end in alongitudinal direction, the calciner comprising: a lowermost oxidantinlet level at which one or more first oxidant inlets are located, andoptionally one or more second oxidant inlet levels located above thelowermost oxidant inlet level in the longitudinal direction and at whichone or more second oxidant inlets are located, said first oxidant inletsand, if present, said second oxidant inlets being connected to a sourceof calciner oxidant having an oxygen content of at least 30% vol; alowermost fuel inlet level at which one or more first fuel inlets arelocated; a flue gas outlet located at the top end of the calciner; aflue-gas recycle inlet located at the bottom end of the calciner; andone or more raw meal inlets, wherein: the lowermost oxidant inlet levelis located above the lowermost fuel inlet level at an oxygen-lean zonedistance Do>0 from said lowermost oxidant inlet in the longitudinaldirection, at least one raw meal inlet being located below or at thelowermost oxidant inlet level in the longitudinal direction, or thelowermost fuel inlet level is located above the lowermost oxidant inletlevel at a fuel-lean zone distance Df>0 from the lowermost oxidant inletlevel in the longitudinal direction, at least one raw meal inlet beinglocated below or at the lowermost fuel inlet level in the longitudinaldirection.
 2. The calcination installation of claim 1, whereby the anoxygen-lean zone distance Do, respectively the fuel-lean zone distanceDf is between 1/10 and 4/10 of the total calciner height.
 3. Thecalcination installation of claim 1, further comprising a raw mealpreheater connected to the flue gas outlet and to the one or more rawmeal inlets of the calciner.
 4. The calcination installation of claim 3,further comprising a flue gas purification installation connected to theflue gas outlet of the calciner, said flue gas purification installationbeing adapted for removing components other than CO₂ from flue gasevacuated from the calciner via the flue gas outlet.
 5. A cement clinkerproduction unit comprising a calcination installation of claim 1 and aclinkerisation kiln, the clinkerization kiln being connected to thecalcination installation so that meal calcined in the calciner istransferred to the clinkerization kiln.